Center drilling hold-down device

ABSTRACT

An apparatus for automatically aligning a hydraulic drill with the longitudinal axis of a piece of stock material so that a centering hole may be drilled therein to facilitate subsequent machining of the stock is disclosed. More particularly, the apparatus includes a linkage comprising a pair of horizontally displaced bars coupled to a substantially flat frame member so that each is pivotable in a vertical plane normal to the frame. A second, shorter pair of pivoted bars is similarly positioned directly below the first pair. The free-moving end of each short bar, in turn, is coupled to an intermediate point on the corresponding by a vertical bar so that the linkage forms a pair of parallelograms. A hold-down means, attached to the free-moving ends of the longer outwardly extending bars, is positioned to contact the upper surface of the stock as it rests in a V-shaped stock support so that the stock is secured in position for drilling with its end surface parallel to the frame. A drill is mounted between the vertical bars so that it is normal to the end of the stock. Accordingly, when the holddown contacts the top surface of the stock, the drill is automatically aligned with the longitudinal axis of the stock so that a centering hole can be drilled.

United States Patent 91 Lanman CENTER DRILLING HOLD-DOWN DEVICEInventor: Robert W. Lanman, Munster, Ind. I

Assignee: Hadady Machining Co., Inc.,

Lansing, Ill.

Dec. 21, 1972 Filed:

Appl. No.:

US. Cl. 408/104, 408/234 Int. Cl B23b 45/14 Field of Search 408/105,106, 104, 103,

[5 6] References Cited UNITED STATES PATENTS l/l890 Minnich 408/1063/1910 Merrill 408/107 X l/1949 Bowen 408/105 X Primary Examiner-AndrewR. Juhasz Assistant Examiner-Z. R. Bilinsky Attorney, Agent, or Firm-Merriam, Marshall, Shapiro & Klose [57] ABSTRACT a An apparatus forautomatically aligning a hydraulic July 23, 1974 drill with thelongitudinal axis of a piece of stock material so that a centering holemay be drilled therein to facilitate subsequent machining of the stockis disclosed. More particularly, the apparatus includes a linkagecomprising a pair of horizontally displaced bars coupled to asubstantially flat frame member so that each is pivotable in a verticalplane normal to the frame. A second, shorter pair of pivoted bars issimilarly positioned directly below the first pair. The freemoving endof each short bar, in turn, is coupled to an intermediate point on thecorresponding by a vertical bar so that the linkage forms a pair ofparallelograms. A hold-down means, attached to the free-moving ends ofthe longer outwardly extending bars, is positioned to contact the uppersurface of the stock as it rests in a V-shaped stoclc support so thatthe stock is secured in position for drilling with its end surfaceparallel to the frame. A drill is mounted between the vertical bars sothat it is normal to the end of the stock. Accordingly, when theholddown contacts the top surface of the stock, the drill isautomatically aligned with the longitudinal axis of the stock so that acentering hole can be drilled.

17 Claims, 7 Drawing Figures PATENIEDJULBWH 3.825.363

sumzum I HIM flai ni 'PATENTEU LZ 3.825.363 SHEET 30E 4 FIG. 6

CENTER DRILLING HOLD-DOWN DEVICE BACKGROUND OF THE INVENTION The presentinvention relates generally to a device for determining the center of asymmetric cross sectional area and more particularly to a device forsimultaneously holding stock material of symmetric cross sectional areain a stationary position and aligning a drilling apparatus to becoincident with its longitudinal axis so that a centering hole may bedrilled therein, facilitating subsequent machining of the stock.

Reference may be made to the following U.S. Pat. Nos. 2,810,310;2,779,040; 2,678,572; 1,292,620.

When a piece of stock material is to be machined on a lathe, centeringholes are commonly provided at each end coincident with its longitudinalaxis. The centering holes perform a twofold function. That is, theturning pins of the lathe are inserted into the centering holes tosupport the stock as it is rotated by the lathe. Secondly, accurateplacement of the centering holes insures that the mass of the stockspiece will not be disproportionately distributed around the turning axisdetermined by the centering holes, and consequently, vibrations will beminimized. If, on the other hand, the holes are not accurately centered,it follows that the resulting vibrations make machining more difficultand, if continued, are likely to damage the lathes bearings.Accordingly, the centering holes should be determined with as muchaccuracy as possible.

When only stock pieces having identical cross sections are to be centerdrilled, the holesmay be placed with consistant accuracy by clamping thestock in the same position for each piece, aligning a drilling apparatuswith the longitudinal axis and limiting the drills movement to thataxis. Where stock of different diameter is intermixed, however, thedrill must be realigned with the longitudinal axis of each stock piece.Of course, this requires additional time to determine the exact centerof the stock. Accordingly, machines have been developed which clamp thestock in position and automatically align a drilling apparatus with thelongitudinal, or center axis of the stock.

One such prior art machine. utilizes a pair of V- shaped jaws; one jawproviding a stationary base for supporting the stock in a horizontalposition and a second, movable, jaw for contacting the top of the stockto clamp it in position for drilling. A pair of vertical columns,positioned on opposite sides of the stock, are connected to the movablejaw; and a threaded nut assembly is affixed between the bottom ends ofthe vertical columns. The nut, in turn, engages a stationary, butrotatable, threaded rod which controls the vertical movement of the nutand, hence, the movable jaw. Thus, as the threaded rod is manuallyrotated, the nut is translated along the threaded rod until the movablejaw contacts the top of the stock. The nut is also linked to one end ofa lever, the opposite end of which is similarly linked to a fixed pointon the machine frame. A vertical bar is coupled to the middle of thelever and a belt-driven drill mounted thereon. Accordingly, any movementof the top, or movable, jaw is translated through the vertical columnsand the nut assembly to the lever so that the drill is moved one-halfthe distance that the movable jaw travels. Thus, if the drill isinitially positioned midway between the jaws, it will be properlyrepositioned when the top jaw is moved to accommodate stock of differentdiameters.

In a prior art machine of this type, however, lateral movement of thevertical bar supporting the drill must be minimized to prevent verticaldisplacement of the drill from the longitudinal axis. Thus, the verticalcolumns are constrained to move only in the vertical direction so thatthe nut assembly can not be twisted and thereby impart lateral movementto the lever and the vertical bar. To provide the requisite structuralrigidity, the vertical columns are passed through bores, or guides,machined to close tolerances for preventing lateral movement of thevertical columns. Accordingly, the bores must be periodically lubricatedto prevent friction wear.

Another disadvantage is that the vertical columns, nut assembly, lever,vertical bar, etc., must meet precise physical specifications if thedesired ratio between the displacement of the movable jaw and thepositioning of the drilling apparatus is to be preserved. Any deviationsufficient to change the ratio will cause the centering holes to bedisplaced from the longitudinal axis with expected results. Of course,as greater precision is required, the cost of attempting to achieve andmaintain same also increases.

The prior art machine also utilizes a belt-driven drilling apparatuswhich is highly susceptible to producing vibrations than can damage thedrill bearings and, as expected, adversely affect the useful life andaccuracy of the machine. Also, vibrations create chatter marks in theconical center pocket being drilled in the stock, which adverselyaffects the subsequent machining of the stock.

SUMMARY OF THE INVENTION The center drilling apparatus of the presentinvention includes means comprising a parallelogram linkage having apair of vertically displaced members pivotally coupled to a frame andconstrained to move within a vertical plane normal thereto. A supportmember, coupled between the two members, is parallel to the surface ofthe frame as well as the end of the stock. Holddown means secure thestock in position during drilling by contacting the upper surface of thestock as it lies horizontally in the plane of the parallelogram linkage.More particularly, the hold-down means is attached to the end of the topmember which extends beyond the support member for a predetermineddistance. Accordingly, drill means, mounted on the support member normalto the end of the stock, are automatically aligned with the longitudinalaxis of the stock whenever the hold-down means is in contact with itsupper surface.

As a further feature of the invention, means are provided for advancingthe drill means toward the end surface of the stock to facilitatedrilling therein. Also, bydraulic means are attached to the end of thetop outwardly-extending member so that proper contact between thehold-down means and the surface of the stock is accomplished. ,Anadditional feature is the provision of drill means powered by ahydraulic motor thereby reducing the vibration commonly associated withbelt-drive motors.

I BRIEF DESCRIPTION OF THE DRAWINGS The features of this invention whichare believed to be novel are set forth with particularity, in theappended claims. The invention together with its further objects andadvantages thereof, may be best understood, however, by reference to thefollowing description taken in conjunction with the accompanyingdrawings, in which like reference numerals identify like elements in theseveral figures and in which:

FIG. 1 is a view in perspective of a center drilling hold down device inaccordance with a preferred embodiment of the invention;

FIG. 2 is a side elevational view of the center drilling hold downdevice of the present invention;

FIG. 3 is an end view of the center drilling hold down device of thepresent invention;

FIG. 4 is another side elevational view of the center drilling hold downdevice as it appears when-it has changed position to center drill smalldiameter stock;

PREFERRED EMBODIMENT OF THE INVENTION view of the pivot taken along lineReferring now to FIG. 1, there is shown a center drilling apparatus inaccordance with the present invention. More particularly, the machineincludes a vertical frame member 11 having a pair of base members 12affixed thereto and extending outwardly parallel to one another in thehorizontal plane. Several cross members 13 are attached to the basemembers 12, lying perpendicularly across the base members 12 to providea rigid platform, or base, upon which a tongued plate 14 is mounted-AV-shaped stock support 15 having a corresponding groove for slidablyinterlocking the tongued plate 14 is especially well suited to stablysupport any stock of symmetric cross sectional area, e.g., round,square, hexagonal. An auxiliary V-shaped stock support 16 may likewisebe slidably fitted on tongued plate 14 at a point distant from support15 so that stock material of any desired length maybe accommodated. Alip 17 is provided on support 15 to provide a stop against which thestock material may be butted during the drilling operation. Accordingly,the cross sectional surface of the stock to be center drilled may bemaintained in a horizontally stable position. v

A mounting plate 18 is affixed to the frame 11 for mounting theparallelogram linkage and drilling assembly of the present invention.More particularly, the plate 18 has four mounting-flanges 19, eachlocated at a respective corner of an imaginary rectangle drawn on theface of mounting plate 18. A pair of bars 20 are coupled to theupper-most two flanges 19 by pivots 21 so that the bars 20 are pivotablewithin an imaginary plane extending outwardly from the vertical framemember 11 through the base members 12. The bars 20 are joined at theirfree-moving ends by a spacing bar 22 affixed to the bars 20 by pivots 23about which the spacing bar 22 may rotate. A hold-down plate 24 isfastened to the underside of spacing bar 22 by means of a set screw 25.Accordingly, if pieces of stock having different diameters are to bedrilled, the hold-down plate 24 is free to pivot so that it may freelycontact the top surfaceof the stock to be drilled at any particulartime.

The free-moving ends of bars 20 are, in turn, coupled to a correspondingpair of hydraulic cylinders 26 through a pair of connecting rods 27 eachof which passes thru alateral stabilizer bracket 30. One end of eachconnecting rod 27 ends in a U-shaped bracket 28 which further accepts apin 29 passing through a hole in the end of rod 20. Although a gravityoperated holddown arrangement could be used, the hydraulic cylinders 26are provided to insure that the hold-down plate 24 continually contactsthe top surface of the stock material in each instance so that thecenter hole may be accurately placed. An incidental advantage is alsoderived in that the hydraulically positioned hold-down plate 24 furtherserves to insure that the stock material is securely held in placeduring the drilling operation.

A pair of shorter outwardly extending bars 31 are, in turn, coupled tothe bottom pair of flanges 19 by pivots 21. Further, two correspondingvertical bars 32 are positioned so that one end of each bar 32 iscoupled to an intermediate point on the corresponding bar 20 by pivot 33while its other end is coupled to the end of bar 31 by pivot 33. Thevertical bars 32 are placed so that they lie in a vertical plane lyingparallel to the frame 1 1. Accordingly, the bars 20, 31 and 32 combinewith the frame 1 1 to provide a parallelogram linkage comprising a pairof parallelograms, each having twocorners permanently fixed in astationary position.

A plate, or web member, 34 extending between th vertical bars 32accommodates a hydraulic drill 35 which may be slidably fitted throughan aperture in plate 34 so that the drill bit is coincident with thevertical plane passing thru the longitudinal axis of the stock. Aslotted rack 36 is affixed to the bottom of the hydraulic drill 35 sothat it is in contact with a pinion-gear 37 mounted on an axle 38. Inturn, the axle 38 is mountedthrough holes in one of the vertical bars 32and a flange 39 extending downwardly from plate34. Consequently, therack 36 and hence the hydraulic drill 35 may be moved in the plane ofthe longitudinal axis of the stock resting in the V-shaped support 15 byturning a wheel 40 having a handle 41 that is attached to axle 38 forrotation pinion 37.

During the drilling of center holes, the hydraulic drill 35 would besusceptible to twisting within the aperture of plate 34 as the bit comesin contact with the stock. Accordingly, a pair of plates 42 are affixedto the hydraulic drill 35 at points on either side of the plate 34. Thetwo plates 42, are in turn, connected by rods 43 passing through holesin plate 34 so that the hydraulic drill 35 is not rotatable within itsaperture. The holes are dimensioned such that the rods 43 pass freelythe rethrough. A center drill bit 44 is mounted on the hydraulic drill35 and rotated thereby so that it is effective to center drill a hole atthe center of the cross sectional area of stock material held in theV-shaped support 15.

bar 45 by pivot 47 and to a flange 48 on the spacing bar 22 by pivots47. Accordingly, a second parallelogram arrangement is provided formaintaining the hold-down plate in a perfectly horizontal position nomatter what are the free-moving ends of rods 20 may be moved through.

The operation of the center-drilling apparatus may be more easilyunderstood by reference to FIG. 2. There it may be seen that thehydraulic drill 35 is mounted between the vertical bars 32 in spatialrelationship with the bottom surface of hold-down 24 so that the centerdrill bit 44 is aligned with the longitudinal axis of the stock when thehold-down 24 contacts the top surface thereof. Accordingly, when thecenter hole is to be drilled, the rack 36 and pinion 37 are actuated tomove the hydraulic drill 35 toward the stock coincident with itslongitudinal axis so that a center hole can be drilled. Once the centerhole is drilled, the combined action of the rack 36 and the pinion 37withdraws the center drill bit 44 and the hydraulic drill assembly 35 sothat the stock may be removed and the next piece placed in the stocksupport for center drilling.

The lateral displacement, or positioning, of the vertical bars 32 alongthe bars is mathematically determined so that the center drill bit 44remains aligned with the longitudinal axis of the piece as the diameterof the stock changes. FIG. 3 is an end view of the center drillinghold-down device as it appears in FIG. 2. There a piece of round stockmaterial having'a radius R is shown resting in the V-shaped stocksupport 15. Referenced to the base of the V groove, which is identifiedgenerally as reference, it may be seen that the distance between thecenter, or longitudinal, axis of the stock and reference is a distanceequal to R (2)" and accordingly, the distance to the top of the stockmaterial is R [1 (2)" Thus in FIG. 2 when the parallelogram linkage isrotated about pivots 21 so that hold-down 24 contacts the top surface ofstock, it may be seen that the pivots 23 define an are at a constantradius (A B) about pivots 21 and that pivots 33 likewise define an areabout pivot 21 having a radius equal to B. Accordingly, regardless ofvariations in the diameter of the stock material the hold-down 24 willalways be at a distance of R,[l (2)] from reference, and the centerdrill bit 44 should bear the relationship R ,(2)" to the reference."Consequently, the optimum lateral displacement of the vertical bars 32along the bars 20 may be determined by the ratio:

where AR R,,R, R and R being the respective radii of two pieces ofstock.

Accordingly, the distance (A) at which the pivots 33 are displaced frompivots 21 should be:

Thus, when the hold-down 24 is moved away from the reference, thehydraulic drill 35 will likewise be repositioned to the center point, orlongitudinal axis, of the stock. Although the free-moving ends of bars20 define arcs around the pivots 21, it should be noted that thevertical bars 32, because of the parallelogram linkage, are always inthe vertical plane. Hence, the hydraulic drill 35 is always normal tothe end of the stock.

FIG. 4 illustrates the center drilling apparatus as it appears when acenter hole is to be drilled in stock having a small diameter. A pair ofslots 49 are provided in the V-shaped stock support 15 to accommodatethe spacing bar 22 so that the hold-down plate 7 24 can contact theupper surface of the stock positioned therein. Altematively, thehold-down 24 may be extended downward from the spacing bar 22 for somedistance so that extremely small diameter material can be center drilledeven though the spacing bar 22 is limited to movement above the top ofstock support 15.

Referring now .to FIG. 6, there is shown a sectional view of the pivot33 shown in FIG. 5. As shown, the pivot assembly 33 utilizes taperedroller bearings 50 to provide a nearly frictionless rolling surface apivot vertical bar 32 relative to bar 20 with great rigidity andstability. The tapered roller bearings 50 are placed between an innercone 51 and an outer cone 52 which are in contact with a bolt 53 and bar20, respectively. Thus,

view taken along line 7-7 in FIG. 5. An important advantage realized byutilizing tapered roller bearings over any other form of roller bearinglies in the ready provision for minimizing the effect of wear whichcould decrease lateral rigidity of the parallelogram'linkage.

Accordingly, there has been shown a center drilling apparatus inaccordance with the present invention which may be utilized for drillingcentering holes in stock of different size and symmetric cross-sectionalshape which are intermixed with one another. Thus, there is no need toreadjust the machine to the center point, or longitudinal axis, of thestock each time stock of different size or shape is to be drilled.Rather, the machine of the present invention automatically aligns itsdrilling apparatus with the longitudinal axis of the stock whenever thehold-down means is placed in contact with the upper longitudinal surfaceof the stock to secure it for drilling. Further, the center drillingmachine of the present invention utilizes a hydraulic motor to power thedrill bit thereby reducing the vibrations commonly associated with theprior art belt driven drilling machines. Since the parallelogram linkageof the present invention is inherently more rigid than the prior artmachines previously discussed and since tapered roller bearings are usedat the pivot points to increase the rigidity thereof, vertical guidesare not required to insure that the drill bit is correctly aligned withthe longitudinal axis of the stock. Rather, in the present invention,the hydraulic vertical guides are used merely to insure that thehold-down device is clamped on to the stock. Consequently, the verticalguides need not be machined to close tolerances or lubricated therebyreducing the maintenance cost of the machine. I

While a particular embodiment of the present invention has been shownand described, it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from theinvention in its broader aspects. For example, the apparatus need notnecessarily be disposed or employed in a generally horizontal drillingposition but may be used in any drilling plane. Accordingly, the aim inthe appended claims is to cover all such changes and modifications asshould fall within the true spirit and scope of the invention.

I claim:

1. An apparatus for drilling a centering hole in the end surface ofapiece of stock coincident with the longitudinal axis of said stock, saidapparatus comprising in combination:

a stationary frame member;

means including a parallelogram linkage pivotably connected to saidframe member and constrained to move in a plane coincident with saidlongitudinal axis, said linkage comprising a pair of parallel alignedmembers pivotably connected to said frame member and a support memberpivotably connected between said parallel members, said support memberbeing intermediate and parallel to said frame member and said stock endsurface, one of said parallel members extending from said frame memberbeyond said support member for a predetermined length to contact saidstock piece immediately adjacent said end surface;

drill means mounted on said support member normal to said stock endsurface for drilling therein, said drill means being automaticallyaligned with said longitudinal axis when said extended parallel membercontacts said stock piece immediately adjacent said end surface.

2. A center drilling apparatus in accordance with 7 claim 1 includinghydraulic hold-down means coupled to said parallelogram linkage forpressing said extended parallel member against said stock to securelyposition said stock and align said drill means with said longitudinalaxis during drilling.

3. A center drilling apparatus in accordance with claim 1 includingstock support means for positioning said stock in alignment with saidparallelogram linkage so that said end surface is normal to said drillmeans and at a predetermined distance from said frame member so thatsaid extended parallel member contacts said stock immediately adjacentsaid end surface.

4. A center drilling apparatus in accordance with claim 3 wherein saidstock support means comprises a V-shaped base having a lip at one end,said stock resting in a substantially horizontal position with said endsurface being butted against said lip.

5. A center drilling apparatus in accordance with claim 3 includingauxiliary stock support means positioned in alignment with said stocksupport means and at a distance therefrom to permit longer pieces ofsaid stock to be stably positioned for drilling by supporting thatportion of said stock extending from said stock support means.

6. A center drilling apparatus in accordance with claim 1 wherein saiddrill means includes a hydraulic drill and means for advancing saidhydraulic drill along said longitudinal axis of said stock to drill acentering hole in said stock end surface, said means withdrawing saidhydraulic drill from said stock when center drilling is completed.

7. A center drilling apparatus in accordance with claim 1 wherein saidextended parallel member is substantially l (2) times longer than theother of said parallel members.

8. An apparatus for drilling a centering hole in the end surface of apiece of stock coincident with the longitudinal axis of said stock, saidapparatus comprising in combination:

a frame member;

means including a pivoted parallelogram linkage comprising:

a. a first pair of horizontally displaced bars, each of said first barshaving one end pivotably connected to said frame member and having afree-moving end;

b. a second pair of shorter horizontally displaced bars, each of saidsecond bars having one end pivotably connected to said frame member andhaving a free-moving end, each of said second bars being positionedbelow said first bars in vertical alignment with a corresponding one ofsaid first bars; and

c. a pair of vertical bars, each of said vertical bars having a firstend pivotably connected to said freemoving end of the corresponding oneof said second bars and a second end pivotably connected to thecorresponding one of said first bars intermediate said pivoted end andsaid free-moving end thereof, said vertical bars being intermediate andparallel to said frame member and said stock piece end surface, saidparallelogram linkage being constrained to move in a plane substantiallycoincident with said longitudinal axis;

hold-down means for interconnecting said freemoving ends of said firstbars; and

drill means mounted between said vertical bars normal to said stockpiece end surface for drilling therein, said drill means being alignedwith said longitudinal axis when said hold-down means is positioned onsaid stock piece immediately adjacent said stock piece end surface.

9. A center drilling'apparatus in accordance with claim 8 includinghydraulic means attached adjacent said free-moving ends of said firstbars to move said hold-down means into contact with said stockimmediately adjacent said end surface for securing said stock inposition forv drilling and aligning said drill means with saidlongitudinal axis.

10. A center drilling apparatus in accordance with claim 8 includingstock support means for positioning said stock in alignment with saidparallelogram linkage so that said end surface is normal to said drillmeans and at a predetermined distance from said frame member so thatsaid hold-down means contacts said stock immediately adjacent said endsurface.

11. A center drilling apparatus in accordance with claim 10 wherein saidstock support means comprises a V-shaped base having a lip at one end,said stock resting in a substantially horizontal position with said endsurface being butted against said lip.

12. A center drilling apparatus in accordance with claim 11 wherein saidV-shaped base includes slots therein aligned with said hold-down meansfor accommodating said hold-down means when said stock is of a smalldiameter.

13. A center drilling apparatus in accordance with claim 8 including asubstantially flat plate affixed between said vertical bars for mountingsaid drill means normal to said stock end surface and in a planecoincident with said longitudinal axis of said stock, said plate havingan aperture through which said drill means is slidably mounted.

14. A center drilling apparatus in accordance with claim 13 includingmeans for preventing said drill means from twisting within said aperturewhen said claim 8 including second linkage means for maintaining saidhold-down means substantially parallel to the upper surface of saidstock when said hold-down means is not in contact with said stock.

17. A center drilling machine in accordance with claim 8 including pivotmeans comprising tapered roller bearings for pivotably interconnectingsaid first bars, said shorter second bars, said vertical bars and saidframe member.

-'P0-1 9 UNITED STATES PATENT OFFICE (s/ss) i CERTIFICATE OF CORRECTIONlatent No., 5 363 Dated Julv 23 1 974 Inventofls) Robert W. Lanman It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

FXB STRACT [57] righthand column, Line 11, after "corresponding" insertlonger bar-; Line 14, insert hyphen between --outwardly extending;

Column 3, Line 18, "views" should be -view-- Celumn 6, Line 11, change"a pivot" to to pivot---;

Column 7, Line 62, Claim 7,' formula should read -l+ (2) Signed andsealed this 8th day of Cctober (SEAL) Attest. v

McCOY M; GIBSON JR. c. MARSH ALL DANN Attesting Officer Commissioner ofPatents

1. An apparatus for drilling a centering hole in the end surface of apiece of stock coincident with the longitudinal axis of said stock, saidapparatus comprising in combination: a stationary frame member; meansincluding a parallelogram linkage pivotably connected to said framemember and constrained to move in a plane coincident with saidlongitudinal axis, said linkage comprising a pair of parallel alignedmembers pivotably connected to said frame member and a support memberpivotably connected between said parallel members, said support memberbeing intermediate and parallel to said frame member and said stock endsurface, one of said parallel members extending from said frame memberbeyond said support member for a predetermined length to contact saidstock piece immediately adjacent said end surface; drill means mountedon said support member normal to said stock end surface for drillingtherein, said drill means being automatically aligned with saidlongitudinal axis when said extended parallel member contacts said stockpiece immediately adjacent said end surface.
 2. A center drillingapparatus in accordance with claim 1 including hydraulic hold-down meanscoupled to said parallelogram linkage for pressing said extendedparallel member against said stock to securely position said stock andalign said drill means with said longitudinal axis during drilling.
 3. Acenter drilling apparatus in accordance with claim 1 including stocksupport means for positioning said Stock in alignment with saidparallelogram linkage so that said end surface is normal to said drillmeans and at a predetermined distance from said frame member so thatsaid extended parallel member contacts said stock immediately adjacentsaid end surface.
 4. A center drilling apparatus in accordance withclaim 3 wherein said stock support means comprises a V-shaped basehaving a lip at one end, said stock resting in a substantiallyhorizontal position with said end surface being butted against said lip.5. A center drilling apparatus in accordance with claim 3 includingauxiliary stock support means positioned in alignment with said stocksupport means and at a distance therefrom to permit longer pieces ofsaid stock to be stably positioned for drilling by supporting thatportion of said stock extending from said stock support means.
 6. Acenter drilling apparatus in accordance with claim 1 wherein said drillmeans includes a hydraulic drill and means for advancing said hydraulicdrill along said longitudinal axis of said stock to drill a centeringhole in said stock end surface, said means withdrawing said hydraulicdrill from said stock when center drilling is completed.
 7. A centerdrilling apparatus in accordance with claim 1 wherein said extendedparallel member is substantially 1 + (2)1/2 times longer than the otherof said parallel members.
 8. An apparatus for drilling a centering holein the end surface of a piece of stock coincident with the longitudinalaxis of said stock, said apparatus comprising in combination: a framemember; means including a pivoted parallelogram linkage comprising: a. afirst pair of horizontally displaced bars, each of said first barshaving one end pivotably connected to said frame member and having afree-moving end; b. a second pair of shorter horizontally displacedbars, each of said second bars having one end pivotably connected tosaid frame member and having a free-moving end, each of said second barsbeing positioned below said first bars in vertical alignment with acorresponding one of said first bars; and c. a pair of vertical bars,each of said vertical bars having a first end pivotably connected tosaid free-moving end of the corresponding one of said second bars and asecond end pivotably connected to the corresponding one of said firstbars intermediate said pivoted end and said free-moving end thereof,said vertical bars being intermediate and parallel to said frame memberand said stock piece end surface, said parallelogram linkage beingconstrained to move in a plane substantially coincident with saidlongitudinal axis; hold-down means for interconnecting said free-movingends of said first bars; and drill means mounted between said verticalbars normal to said stock piece end surface for drilling therein, saiddrill means being aligned with said longitudinal axis when saidhold-down means is positioned on said stock piece immediately adjacentsaid stock piece end surface.
 9. A center drilling apparatus inaccordance with claim 8 including hydraulic means attached adjacent saidfree-moving ends of said first bars to move said hold-down means intocontact with said stock immediately adjacent said end surface forsecuring said stock in position for drilling and aligning said drillmeans with said longitudinal axis.
 10. A center drilling apparatus inaccordance with claim 8 including stock support means for positioningsaid stock in alignment with said parallelogram linkage so that said endsurface is normal to said drill means and at a predetermined distancefrom said frame member so that said hold-down means contacts said stockimmediately adjacent said end surface.
 11. A center drilling apparatusin accordance with claim 10 wherein said stock support means comprises aV-shaped base having a lip at one end, said stock resting in asubstantially horizontal position with said end surface being buttedagainst said lip.
 12. A center drilLing apparatus in accordance withclaim 11 wherein said V-shaped base includes slots therein aligned withsaid hold-down means for accommodating said hold-down means when saidstock is of a small diameter.
 13. A center drilling apparatus inaccordance with claim 8 including a substantially flat plate affixedbetween said vertical bars for mounting said drill means normal to saidstock end surface and in a plane coincident with said longitudinal axisof said stock, said plate having an aperture through which said drillmeans is slidably mounted.
 14. A center drilling apparatus in accordancewith claim 13 including means for preventing said drill means fromtwisting within said aperture when said drill means engages said endsurface.
 15. A center drilling apparatus in accordance with claim 8wherein said drill means includes a hydraulic drill having a rack gearattached thereto and a pinion gear mounted on said parallelogramlinkage, said pinion gear engaging said rack gear so that rotation ofsaid pinion gear selectively advances and withdraws said hydraulic drillwith respect to said stock end surface.
 16. A center drilling apparatusin accordance with claim 8 including second linkage means formaintaining said hold-down means substantially parallel to the uppersurface of said stock when said hold-down means is not in contact withsaid stock.
 17. A center drilling machine in accordance with claim 8including pivot means comprising tapered roller bearings for pivotablyinterconnecting said first bars, said shorter second bars, said verticalbars and said frame member.